Typical evacuation slides comprise both an aspirator for filling the slide and a pressure relief valve (PRV) for exhausting excess gas pressure as part of their inflation systems. An aspirator usually allows the inflatable to intake gas for inflation and to retain gas after inflation. A PRV typically provides an outlet for gas in the event pressure inside the inflatable exceeds a predetermined threshold.
In an aircraft, available volume and weight capacity come at a premium. A common problem with evacuation systems is pack density. Pack density refers to the amount of packed slide and components occupied in response to the system being packed into the space available. Pack weight is an additional consideration. The aspirator and PRV are typically separate components. As such, each occupies its own separate share of the overall allowable pack space.